Imagine you and your friends are all miners who took out $50k loans to buy nice mining gear in the Fall of last year. You're the one who mines and only sells to cover his fiat loan. You interact with these friends and colleagues every day, and they all talk about how they've been selling everything as they mine it.
Your first month of mining was in December of 2013 and after selling to cover fiat expenses, you made a profit of 50 BTC.
Your second month of mining was in January of 2014 and you made 40 BTC.
Your third month of mining 30 BTC.
Your fourth month 20 BTC.
You go to a miner's consortium in early April 2014, and your friends talk about how the price has fallen so much, they were glad they were selling. Turns out they made the same kind of profits but sold right away, here's how they worked out...
December 2013: 50 BTC sold at ~$1000 each for $50k
January 2014: 40 BTC sold at $800 each for $32k
February 2014: 30 BTC sold at $600 each for $18k
March 2014: 20 BTC sold at $500 each for $10k
Total in Fiat: $110k
So time comes around to where they ask you how you've been doing. You say, fine, things have been going great, but you're thinking it's time to shut down here in a few months, you're not sure of the future profitability. Your friends turn to you and say they actually have plans to close out their loans and liquidate their hardware next week and just pocket their profits, too. They owe about the same amount, $50k for the mining equipment, and they ask you if you'll be closing out your loan, too.
You made a handsome 140 BTC, but you realize that in order to finish off the loan, you'll need to cash out of nearly all of it at ~$400 ($56k), and you'll pocket a whopping $6k in profits. You'll get a lot of "oh man that's too bad" from your friends, but at the end of the day, they'll be the ones walking away with the bigger profits.
Yes, you could cover loan costs in fiat from a day job, but you're still realizing that the guys who sold made more money. Now, imagine this same Miner's consortium among you and your friends happened in January instead. Would you still have not sold immediately in February and March?
TL;DR: Pressure comes to sell eventually for miners, especially if they have loans. The market can remain irrational longer than you can remain solvent.